Machine failure has been a major concern of industrial operators for well over a century. Machine failure typically results from a degradation mechanism inherent in the operation of the machine (e.g., vibration, erosion, corrosion, cavitation, etc.). Ultimately, this degradation mechanism causes the machine to fail, thereby requiring replacement or repair of the machine or its components. Because machine repair often creates significant economic losses (in terms of both actual repair costs and lost operating time), machine operators desire to minimize the frequency at which machine components fail while also minimizing the cost of operation. To minimize the operation and maintenance costs, a number of different maintenance regimes may be employed.
A first, and most basic, maintenance regime is corrective maintenance. Corrective maintenance involves running a piece of machinery until it fails. Therefore, corrective maintenance is reactive in nature, and little, if any, attention is paid to ensuring that the operating conditions are within the design envelope. Consequently, the life span of the equipment is typically below the estimates of the manufacturer. Corrective maintenance requires little forethought and uses the least amount of resources from the operating and maintenance crew, at least until the machinery fails. There are numerous examples of how equipment is destroyed by rapidly acting degradation mechanisms. For example, erosion or cavitation in a pump can shorten the life expectancy of the pump by an order of magnitude or more. In very simple, non-critical components (e.g., a light bulb), the corrective-maintenance regime may be a cost-effective regime for maintaining equipment. As long as the consequence of equipment failure is not high, this approach has merit. However, in critical applications, such as the safety systems of a nuclear power plant, this risky approach is not tolerable.
FIG. 1A illustrates the prior art corrective-maintenance regime and the other prior art maintenance schemes discussed herein. The operational lifetime of a centrifugal pump is used to illustrate the various regimes. In FIG. 1A, a new pump 10 is put into service and is run until it fails at time period 12. At time period 14, a degradation mechanism 16, which had been active since the installation of the pump, begins to become noticeable by its effects on the performance of the pump 10. Because the corrective-maintenance regime involves running the machinery until failure, the time period 18 associated with corrective maintenance does not begin until after failure of the pump. By way of example, the lifetime of a centrifugal pump operating under this regime may be about six months.
A second maintenance regime is preventive maintenance. Preventive maintenance involves periodically checking the performance and material condition of a piece of equipment to determine if the operating conditions and resulting degradation rate are within the expected limits. If the degradation is outside the expected limits, the source of the degradation must be found so that it can be corrected, or at least mitigated, before the machine fails. Preventive testing, inspecting, servicing, and part replacing are done on a service-life (e.g., hours of operation) or time-in-service basis. Although accurate failure statistics can allow the testing interval to be optimized, the preventive-maintenance method is expensive, and catastrophic failures may still occur. Moreover, the preventive-maintenance method is very labor intensive and risky. Unnecessary maintenance is often performed, and incidental damage to equipment may occur as a result of poor maintenance practices. Nonetheless, a preventive-maintenance regime may be a cost-effective strategy when the life span of the equipment is well understood and consistent. For example, an air filter in constant use tends to need replacing with a fairly constant frequency.
In FIG. 1A (prior art), the time period when the pump is maintained under the preventive-maintenance regime is the time period 20, which begins when noticeable degradation of the pump occurs. Typically, there are several regular intervals during this time period when the pump is serviced. In comparison with the six-month pump lifetime associated with the corrective maintenance regime, the lifetime of a centrifugal pump operating under the preventive-maintenance regime would be expected to exceed twenty-four months.
A third maintenance regime is predictive maintenance. Predictive maintenance involves taking measurements targeted at the early detection of degradation mechanisms, thereby allowing the resulting degradation to be understood and eliminated or controlled prior to the physical deterioration of the equipment. Many nonintrusive measurement methods are known that allow for the early detection of machine degradation. For instance, vibration analysis, oil analysis, thermography, and ultrasonic analysis may be used to detect the early signs of degradation. The root cause for the degradation can sometimes be identified from these measurements, allowing mitigation efforts to be better targeted.
In FIG. 1A (prior art), the time period when the pump is maintained under the predictive-maintenance regime is the time period 22, which begins before noticeable degradation of the pump occurs because of the ability of the diagnostic systems to detect and predict the onset of the degradation mechanism. However, analysis of the captured data is not simultaneous with its measurement, thereby resulting in a delay between the onset of a stressor condition causing degradation and its mitigation. Accordingly, the time period 22 illustrated in FIG. 1A does not begin immediately upon the activation or onset of pump stressors beyond the design basis. In contrast to the twenty-four-month pump lifetime associated with the preventive-maintenance regime, the lifetime of a centrifugal pump operating under the predictive-maintenance regime would be expected to exceed forty-eight months.
The cornerstone of the predictive-maintenance regime is the accurate measurement of the performance and/or degradation of machinery so that early symptoms of degradation can be detected and corrected. For instance, the most common procedure used in the predictive maintenance regime involves trending an index or parameter that relates to the performance of the equipment. For instance, for a pump, the performance parameter may be the fluid pressure produced at the output of the pump. FIG. 2 (a prior art illustration) shows a performance parameter 30 that starts to decline from its normal operating band (NOB), reaches an alert level, and is subsequently analyzed to try and understand a reasonable projection for residual life. Failure is defined as the point 32 at which the equipment no longer is capable of supporting the function for which it was designed. Associated with this method is a large cone of uncertainty that is created by extending the maximum slope 34 and minimum slope 36 of the predicted trend of the performance parameter. This uncertainty results from inherent uncertainties in the mathematical model used to calculate the predicted trend.
As noted, the known predictive maintenance techniques involve analyzing the collected data at a time after the measurements are made. In some cases, such as the analysis of oil, the time between taking the sample and obtaining the results of the analysis may be significant. Further, as illustrated in FIG. 2, the degree of uncertainty associated with current predictive-maintenance techniques is quite large. Accordingly, even though predictive maintenance has several advantages over the other maintenance regimes, it is still not optimal.